Structures and Properties of Zirconia-Supported Ruthenium Oxide Catalysts for the Selective Oxidation of Methanol to Methyl Formate

The effects of RuOx structure on the selective oxidation of methanol to methyl formate (MF) at low temperatures were examined on ZrO2-supported RuOx catalysts with a range of Ru surface densities (0.2−3.8 Ru/nm2). Their structure was characterized using complementary methods (X-ray diffraction, Raman and X-ray photoelectron spectra, and reduction dynamics). The structure and reactivity of RuOx species change markedly with Ru surface density. RuOx existed preferentially as RuO42- species below 0.4 Ru/nm2, probably as isolated Zr(RuO4)2 interacting with ZrO2 surfaces. At higher surface densities, highly dispersed RuO2 domains coexisted with RuO42- and ultimately formed small clusters and became the prevalent form of RuOx above 1.9 Ru/nm2. CH3OH oxidation rates per Ru atom and per exposed Ru atom (turnover rates) decreased with increasing Ru surface density. This behavior reflects a decrease in intrinsic reactivity as RuOx evolved from RuO42- to RuO2, a conclusion confirmed by transient anaerobic reactions o...